(1) Field of Invention
The present invention relates to a fail-safe device for an automatic transmission system. More particularly, this invention relates to a fail safe device of an automatic transmission system which accomplishes a fixed transmission operation with a vehicle speed and an engine rpm (revolution per minute), when a normal gear shift is not achieved because of malfunction of a sensor equipment or an automatic transmission controller.
(2) Description of the Prior Art
In general, the automobile with automatic transmission automatically achieves a transmission operation through controlling a flow of fluid in the fixed transmission range with the vehicle speed.
To achieve the transmission operation, the transmission controller controls a torque of the fluid through operating a torque converter according to an output power of the engine, outputs a controlling signal for the gear shift according to an operating state of the automobile, and achieves the gear shift through operating the fluid by a valve.
The automobile having the automatic transmission is convenient for the driver because a foot-operated clutch is not required to shift gears and the automobile can be brought to a stop without using the clutch and without shifting the transmission into neutral.
Also, the automobile can easily do a hill climbing departure because of high driving power at a low speed, and is comfortable while driving because the engine torque's delivery is composed of the fluid.
As described above, the automatic transmission system automatically performs the gear shift according to the vehicle driving state. In case a normal gear shift is not achieved because of malfunction of a sensor equipment or an automatic transmission controller, the automatic transmission system provides itself with the fail safe means to prevent a gear shift which has no relation to the current driving state.
If there is a malfunction in the sensor equipment or the automatic transmission controller, the automatic transmission controller changes an output of each solenoid-valve into reset, and achieves a vehicle driving operation having the third speed.
However, by the fail safe function, in case the automatic transmission controller controls the driving operation through the fixed transmission stage which has no relation to the vehicle driving state of the automobile, the temperature of the working oil is abruptly increased because a slip occurs in the torque converter at the time the automobile is started.
The high temperature of the working oil deteriorates operation of oil pressure controlling system. As a result of the foregoing, there is a problem of difficulty in starting of the automobile.